A METHOD AND AN IMPROVED FLUE GAS DUCT IN A STEAM GENERATOR FOR PREVENTION OF ASH ACCUMULATION DURING PART LOAD OPERATIONS

Abstract

This invention relates to a method for prevention of ash accumulation in the flue gas ducts in a steam generator during part load operations, comprising; providing a partition plate (1) across the entire length of the gas duct from the Regenerative Air Preheater (RAPH) to the Electrostatic Precipitator (ESP) in such a way that the cross section of the duct is divided in a ratio of 60% / 40%, each partition being provided with a gate and actuator (G3A1, G3B1); administering flue gas through one or both the partitions (1) of the duct depending on the operating load. The invention further relates an improved flue gas duct in a steam generator.

Full Text

FIELD OF INVENTION
The present invention relates to a method and device for prevention of ash
accumulation in flue gas ducts in a steam generator during part load operation of
the boilers delivering steam to the generator.
BACKGROUND OF THE INVENTION
The utility and industrial boilers are designed to deliver the required steam to
Turbine Generator set to generate electric power. Normally the boilers are
operated at the maximum continuous rating (MCR) capacity. Hence all the
associated systems and products are designed to meet the MCR capacity. The
fuels used to generate steam in the boiler are coal, oil, and gas independently or
in combination. Since the coal as a fuel is cheaper and available in plenty,
generally it is used as fuel in the boiler. The inherent mineral matter in the coal
is converted as ash after the combustion of burnable matters in the coal. The
ash generated during the combustion process is carried away along with the flue
gas and a portion gets collected in the bottom of the furnace.
The flue gas ducting system is designed such that the ash is not settled in the
ducting at the MCR load. The maximum velocity of the flue gas is limited to
avoid the erosion of the duct. However, if the boiler operates at part loads, the
ash from the flue gas settle down in the ducts. At present the ducting system
and support are designed to take care of only certain quantum of ash loading,
and thus the ash settling issue/problem is not avoided. During operation,

occasionally a, few streams of ducting system out of total number of streams are
operated to minimize the ash accumulation.
One of the zones where the ash normally gets accumulated is the flue gas duct
from the Air Preheater (APH) to the Electrostatic Precipitator (ESP) during part
operation. The huge accumulation of ash sometime leads to failure of the ducts
and its supports.
OBJECTS OF INVENTION
It is therefore an object of the invention to propose a method for prevention of
ash accumulation in the fuel gas ducts in a steam generator during part load
operations.
Another object of the invention is to propose a method for prevention of ash
accumulation in the flue gas ducts in a steam generator during part load
operations, which facilitates operational flexibility at part loads, while taking care
of MCR load operations.
A further object of the invention is to propose an improved flue gas duct in a
steam generator for prevention of ash accumulation.
SUMMARY OF THE INVENTION
According to the invention, the ducting system from APH to ESP is partitioned
such that the ash accumulation is prevented during low load operation of the

boilers. Operating logics of the system is envisaged to have no / minimum ash
accumulation during the transient period from low load to MCR load.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows a plurality of the flue gas ducts disposed between the
Regenerative Air Preheater (RAPH) and the Electrostatic Precipitator (ESP),
without having any partition wall in the duct according to the prior art.
Figure 2 shows the flue gas ducts from the Regenerative Air Preheater (RAPH) to
the Electrostatic Precipitator (ESP), according to the invention, with the partition
wall in the duct.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
As shown in fig - 1, G1, G2 etc denotes the gates of different sizes provided at
the locations. The number and sizes of the equipments like APH and ESP will
vary according to the capacity of the boiler.
The cross section shown at BB is typical and the size of the duct varies as
required with boiler capacity.

Section BB in fig - 2 shows the duct partition plate. The complete length of the
duct from RAPH outlet to ESP inlet is divided into two streams by the partition
plate. G1A, G1B etc denotes the two gates at the location to match the duct size
after partition.
Typically the flue gas ducting system after the Air Pre heater is divided into four
streams to match with the downstream equipment, say for example Electrostatic
precipitator (ESP) (Fig.l). There is an inter- connecting duct at the outlet of the
RAPH. The Flue gas stream velocity is selected so as to avoid ash settlement.
In the proposed invention (Fig.2), the ducting system from RAPH to ESP inlet is
divided into two streams, with a partition plate (1) as shown in section BB of the
sketch. The top portion is 40% of the duct area of the original duct size and
bottom portion is 60% of the area. Two independent gates with actuators (G3A1,
G3B1) of each 40% and 60% streams are provided at the same locations, the
locations being same as of the prior art arrangement. The gates (G3A1, G3B1)
are operable independent of each other. All required stiffening, support
arrangement are provided as per the duct design practice.
Thus, the method and device of the invention enables complete elimination of
the ash accumulation if the low load operation is limited to predicted low load
operation.
Each divided stream/portion (B) is provided with a separate isolation gates with
actuator. The operating logic of the system is as follows. When the boiler
operates at around 40% MCR load, the gate provided in the stream will be kept

open from the start of boiler operation. This stream will be continuous to be in
operation till the load reaches about 50% MCR. Once the load starts exceeding
50% MCR, the 60% stream is be opened and 40% stream is closed. If the unit
load exceeds 70%, both the gates 60%&40% of the stream are fully opened.
Thus the ash accumulation at part loads up to 70% MCR is not an influencing
factor. If units operate above 70% MCR and below 100%MCR, there will be ash
settlement to the tune of lower velocity in relation to the load below 100% and
above 70%.
The improved gas duct facilitates maintenance of gas velocities under varying
load conditions, in the optimum range, below which the ash may tend to
accumulate. The improved gas duct further facilitates maintenance of gas
velocities under varying load conditions, in the optimum range, above which the
gas stream may lead to erosion. The flue gas duct also facilitates flexibility in
operation under varying load conditions.

WE CLAIM
1. A method for prevention of ash accumulation in the flue gas ducts in a steam
generator during part load operation comprising;
dividing the ducting system from RAPH outlet to ESP inlet into two streams;
Characterized in that,
providing a partition plate (1) across the whole length of the duct from
Regenerative Air pre heater (RAPH) outlet to Electrostatic Precipitator (ESP)
inlet in such a way that the cross section of the duct is divided in a ratio of
60% bottom and 40% top portion, each partition being provided with
independent gate with actuators (G3A1, G3B1);
wherein the flue gas is administered through one or both the partitions (1) of
the duct depending on the operating load.
2. An improved flue gas duct in a steam generator for prevention of ash
accumulation during part load operation comprising;
the ducting system from Regenerative Air pre heater (RAPH) outlet to
Electrostatic precipitator (ESP) inlet divided into two streams;
Characterized in that,
a partition plate divides the original duct size into a 40% top portion and 60%
bottom portion when two independent gates with actuators (G3A1, G3B1) of
each 40% and 60% streams are provided at same earlier locations.

ABSTRACT

A METHOD AND AN IMPROVED FLUE GAS DUCT IN A STEAM GENERATOR
FOR PRVENTION OF ASH ACCUMULATION DURING PART LOAD
OPERATIONS.
This invention relates to a method for prevention of ash accumulation in the flue gas
ducts in a steam generator during part load operations, comprising; providing a
partition plate (1) across the entire length of the gas duct from the Regenerative Air
Preheater (RAPH) to the Electrostatic Precipitator (ESP) in such a way that the cross
section of the duct is divided in a ratio of 60% / 40%, each partition being provided
with a gate and actuator (G3A1, G3B1); administering flue gas through one or both
the partitions (1) of the duct depending on the operating load. The invention further
relates an improved flue gas duct in a steam generator.